1. Field of the Invention
The present invention relates to an electrophotography type or electrostatic photography type image forming apparatus such as a copying apparatus, a laser beam printer and a facsimile, and a sheet conveying apparatus to be applied to such an image forming apparatus.
2. Related Background Art
A conventional image forming apparatus will be described with reference to FIG. 12. FIG. 12 is a configurational diagram of a laser beam printer configured as the conventional image forming apparatus. Description will be made of an overall configuration and functions of the laser beam printer.
In FIG. 12, a feed cassette 13 which contains loaded sheets S is disposed in a lower section of a main body of the laser beam printer. The feed cassette 13 is configured to be detachable from and attachable to the main body of the laser beam printer.
Reference numeral 14 denotes a feed roller which feeds the sheets S loaded in the above described feed cassette 13 one by one from an uppermost side. Reference numeral 15 denotes a guide member which guides the sheet S fed by the above described feed roller 14.
Reference numeral 16 denotes a conveying roller and reference numeral 17 denotes a driven runner. The conveying roller 16 and the driven runner 17 constitute a conveying roller pair. Rollers 1 and 2 are a registration roller pair comprising an upper registration roller 1 and a lower registration roller 2 which correct a skew feed of the sheet S by allowing a leading end of the sheet S to strike against a nip portion of the registration roller pair in a stop condition to form a loop of the sheet S, and then rotate at a predetermined timing to convey the sheet S to image forming means.
A process cartridge 80 is detachably mounted in the main body of the printer. Mounted in the process cartridge 80 is a photosensitive drum 8 as well as a cleaner 81, a developing device 82, a primary charger (not shown) or the like which are disposed around the photosensitive drum 8.
Reference numeral 18 denotes a laser scanner which scans a laser beam in correspondence to image information and performs write (electrostatic latent image formation) on the photosensitive drum 8. A latent image formed on a circumferential surface of the photosensitive drum 8 is developed by the developing device 82 into a developer (toner) image. Reference numeral 9 denotes a transfer roller which is in pressure contact with the above described photosensitive drum 8 and transfers the developer (toner) image from the photosensitive drum 8 to the sheet S while the sheet S is passing between the transfer roller 9 and the photosensitive drum 8. The process cartridge 80, the transfer roller 9, a fixing device or the like described later compose the image forming means for forming an image on the sheet.
Reference numeral 19 denotes a conveying guide which leads the sheet S to the fixing device after the image is transferred to the sheet S. The fixing device which functions to fix the image (toner image) transferred to the sheet S has a fixing roller 20, a pressure roller 21, a fixing discharge roller pair 22 or the like. The sheet S is conveyed to a discharge roller pair 24 by a conveying roller pair 23 and discharged outside the printer after image fixing with a surface of the sheet S on which the image is formed facing downward.
Reference numeral 25 denotes a discharge tray which is formed on a top surface of the main body of the printer for loading and holding the sheet S which is discharged outside the printer after the image fixing.
Description will be made of image forming operations of the laser beam printer configured as described above. When image formation is designated from a host computer (not shown), the feed roller 14 first rotates, and the sheets S are fed one by one from the uppermost sheet in the feed cassette 13 and guided by the guide member 15 to the conveying roller pair consisting of the conveying roller 16 and the driven runner 17 and the registration roller pair.
The registration roller pair conveys the sheet S between the photosensitive drum 8 and the transfer roller 9 at a timing registered with an image formed on the photosensitive drum 8 and an image (toner image) is transferred to the sheet S while the sheet S is passing between the photosensitive drum 8 and the transfer roller 9.
After transferring the image, the sheet S is conveyed to the fixing device along a conveying guide 19, and the transferred image is fixed by applying heat and a pressure while the sheet S is passing between the fixing roller 20 and the pressure roller 21.
After fixing the image, the sheet S is conveyed by the fixing discharge roller 22 and further discharged and loaded into the discharge tray 25 by the discharge roller pair 24.
A description will be made of a sheet conveying device in detail with reference to FIGS. 12 and 13. FIG. 13 is a schematic diagram of an image forming station of the laser beam printer shown in FIG. 12.
The sheet S which is fed from the feed cassette 13 by the feed roller 14 passes through the conveying roller pair consisting of the conveying roller 16 and the driven runner 17, and is fed into the nip of the registration roller pair consisting of the upper registration roller 1 and the lower registration roller 2 in a stop state of rotating.
The registration roller pair is under a pressure and stops conveyance of the sheet S at an entrance of the nip, thereby allowing the sheet S to form a loop when a leading end of the sheet S is sent into the nip portion of the registration roller pair.
When the registration roller pair starts rotating once again, the sheet S passes between a first sheet guide member 6 and a second sheet guide member 7 and sent into a nip formed by the photosensitive drum 8 and the transfer roller 9.
However, the above described conventional technique poses problems which are described below. That is, it has become general to print high quality images such as photographic images directly on a thick sheet of paper or the like with printers as laser beam printers are configured to operate at higher speeds and form images of higher qualities.
In FIG. 13, the sheet S which is advanced toward transferring means by the registration roller pair is led into the transferring means by the upper and lower sheet guide members 6 and 7. When the sheet S is led into the transferring means, the leading end of the sheet S strikes against the photosensitive drum 8 and then the sheet S is led into a transfer nip.
When the sheet S is a thick sheet of paper in particular, a strong impact is imparted to the photosensitive drum 8 at a write time with a laser, thereby producing an adverse influence on an image. Furthermore, a halftone image may be disturbed due to vibrations of the drum at a developing position which is a position of a developing roller and the photosensitive drum at an instant at which a leading end of the sheet strikes against the photosensitive drum 8, a position at which the photosensitive drum 8 is irradiated with the laser or a charging position at which the photosensitive drum is charged.
Furthermore, an influence on an image is more remarkable as the higher a speed of a printer, the higher a conveying speed of the sheet.
Furthermore, printers have nowadays high resolutions of 1200 dpi and 2400 dpi for enhancing image qualities. The influence on the image due to the impact is more remarkable at a higher resolution. Furthermore, a sheet which is not the thick sheet of paper may produce an influence on an image.
Furthermore, frequently used nowadays as means for transferring a toner image from a photosensitive drum to a sheet is transferring means which uses a contact rotating type transferring member, or the so-called transfer roller, which is brought into contact with a photosensitive drum, subjected to voltage application, and is used for sandwiching and conveying the sheet in a transfer position which is a nip portion between the photosensitive drum and the transfer roller, thereby electrostatically transferring a toner image from the photosensitive drum to the sheet from viewpoint that the transfer roller provides advantage to simplify and stabilize a sheet conveying path and other advantages at the same time.
Though a resistance value of the transfer roller is adjusted to a value on the order of 1xc3x97106 to 1xc3x971010 xcexa9, a transfer roller which is proposed nowadays is configured by disposing an elastic layer around an outer circumferential surface of an electrically conductive core metal and imparting an electrical conductivity to this elastic layer.
Furthermore, the resistance of the transfer roller is liable to change dependently on temperatures and humidities of atmospheric environments and a change in the resistance of the transfer roller induces improper transfer, explosive splash, paper trace or the like.
In order to prevent the improper transfer, paper trace or the like from being caused due to the change in the resistance of the transfer roller, there is adopted xe2x80x9ccontrol of transferring voltage to be appliedxe2x80x9d which measures a resistance value of the transfer roller and appropriately controls a transferring voltage to be applied to the transfer roller dependently on a measured result.
Description will now be made of a conventional image forming apparatus with reference to FIG. 14. FIG. 14 is a schematic side view of a transfer position used in the conventional image forming apparatus. Reference character S denotes a transfer material used as a sheet, reference numeral 8 denotes a photosensitive drum, reference numeral 50 denotes a registration roller pair consisting of an upper registration roller 1 and a lower registration roller 2, reference numeral 7 denotes a lower transfer guide, reference numeral 6 denotes an upper transfer guide, reference numeral 9 denotes a transfer roller, reference numeral 26 denotes a guide support member and reference numeral 3 denotes a charge eliminating brush.
FIG. 14 shows a conveying path for the transfer material S between the registration roller pair 50 and the transfer position. The conventional image forming apparatus is mostly configured to lead the transfer material S into the transfer position along the photosensitive drum 8 upstream of the transfer nip while raising the transfer material S above a tangential line on the transfer nip with the lower transfer guide 7 and the upper transfer guide 6.
This configuration prevents image splash at a transfer time and if the transfer material S is led into the transfer position along the transfer roller 9, a toner image on the photosensitive drum 8 will be transferred before the toner image enters the transfer position under an influence due to an electric field produced between the transfer roller 9 to which a bias voltage is applied and the photosensitive drum 8, thereby allowing an image splash phenomenon (pre-transfer phenomenon) to occur.
Furthermore, the lower transfer guide 7 which is made of an electrically conductive material is capable of providing an electric field shielding effect. Since a higher splash preventing effect is obtained by disposing the lower transfer guide 7 used for shielding an electric field at a position as close as possible to the transfer roller 9 and the transfer position, the lower transfer guide 7 is configured to be swung by a guide support member 26 as exemplified in FIG. 14 while following a motion of the transfer roller 9 and a system is used to enhance positional precision of the lower transfer guide 7 relative to the transfer roller 9.
However, the conventional technique described above poses problems which are described below. In FIG. 14, the transfer material S which is advanced toward the transfer position by the registration roller pair 50 is led into the transfer position by the lower transfer guide 7 and the upper transfer guide 6.
After the transfer material S is led into the transfer position, a leading end of the transfer material 5 strikes against a position 30 of the photosensitive drum 8 and then the transfer material S is led into the transfer nip.
At this time, the transfer material S which is a thin sheet of paper poses no problem, but the transfer material S which is a thick sheet of paper applies an impact to the photosensitive drum 8. A printer which has a higher speed conveys the transfer material S at a higher speed and produces a larger influence on an image.
Furthermore, printers nowadays have resolutions as high as 1200 dpi and 2400 dpi for obtaining images of high qualities. At a higher resolution, the impact produces a more remarkable influence on an image. Furthermore, even a transfer material which is not a thick sheet of paper may produce an influence on an image.
It is therefore an object of the present invention to provide a sheet conveying apparatus and an image forming apparatus which make stabilization of a sheet conveyance in printing images to various sheets, thick sheets of paper in particular, compatible with images of high qualities.
In order to attain the above described object, the sheet conveying apparatus according to the present invention comprises transferring means which transfers a developer developed as an electrostatic latent image on a photosensitive member to a sheet, conveying means which sandwiches the sheet in a conveying nip formed by a rotary member pair and conveys the sheet to the above described transferring means, a sheet guide member which leads the sheet from the above described conveying means to the transferring means, and conveyance regulating member which regulates a conveying direction of the sheet conveyed by the above described conveying means and brings the sheet into contact with the above described sheet guide member.
Furthermore, the above described conveyance regulating member comprises a guide member which regulates a conveying direction of the sheet conveyed by the above described conveying means and guides the sheet.
Furthermore, the above described conveyance regulating member comprises an elastic guide member and regulates the conveying direction of the sheet by urging the sheet which is in contact with the above described guide member.
Furthermore, the above described elastic guide member comprises a conveyance guide portion which extends from a portion supported by a support member supporting the above described guide member and is in contact with the above described sheet, and at least a portion of the above described conveyance guide portion has elasticity.
Furthermore, the above described elastic guide member is hinged to the support member so as to swing which supports the above described guide member and comprises a spring portion which is flexed when the elastic guide member is brought into contact with the above described sheet.
Furthermore, the above described conveyance regulating member comprises a rotary member which is brought into contact with the above described sheet.
Furthermore, the above described conveyance regulating member has electrical conductivity.
Furthermore, the above described conveyance regulating member comprises an electrically conductive fibrous brush.
Furthermore, the above described sheet guide means comprises a first sheet guide member disposed on a side of the above described photosensitive member and a second sheet guide member disposed on a side opposed to the above described first sheet guide member, and the above described conveyance regulating member is disposed among the above described conveying means and the above described first sheet guide member and second sheet guide member at a position on a side of the above described second sheet guide member.
Furthermore, the above described sheet guide member is made of an electrically conductive material and comprises a sheet tailing end regulating member made of an insulating material which regulates a trailing end of the sheet at an upstream end in a conveying direction of the above described sheet and on a side opposite to the above described conveyance regulating member.
Furthermore, the above described transferring means is a rotary member which sandwiches and conveys the above described sheet while being in contact with the above described photosensitive member.
Furthermore, the image forming apparatus according to the present invention comprises image forming means which forms an image on the sheet conveyed by the above described sheet conveying apparatus using the above described photosensitive member and the above described transferring means.
Furthermore, the image forming apparatus according to the present invention comprises a process cartridge which comprises at least the above described photosensitive member and which is detachably mountable in a main body of the image forming apparatus, and the above described process cartridge comprises the above described sheet guide member.
Furthermore, the present invention has another object to provide an image forming apparatus which is capable of stabilizing a sheet conveyance in forming an image on various kinds of sheets such as a thick sheet and enhancing qualities of formed images.
In order to attain the above described object, the image forming apparatus according to the present invention comprises a pair of conveying members for sandwiching and conveying a sheet on which an image is to be formed, an image bearing member for bearing an image to be formed on the above described sheet, and transferring means to be brought into contact with the above described image bearing member for transferring a toner image on the above described image bearing member to the above described sheet, the image forming apparatus characterized by guide means which guides the above described sheet conveyed by the above described pair of conveying members to a transfer nip of the above described image bearing member and the above described transferring means, and a conveyance regulating member which regulates a conveying direction of the sheet conveyed from a conveying nip of the above described pair of conveying members, in that a straight line passing the conveying nip of the above described pair of conveying members and a sheet contact point on the above described conveyance regulating member intersects with the above described guide means.
Furthermore, the above described guide means comprises a sheet trailing end regulating member which comes into contact with the sheet having passed through the above described conveying nip and regulates a trailing end of the sheet.
Furthermore, the above described sheet trailing end regulating member is made of an electrically insulating material.
Furthermore, the above described transferring means is a transfer roller which comprises a support means which is hinged so as to swing by a rotary shaft of the above described transfer roller and urged in a predetermined direction by elastic means, and the above described guide means is formed integrally with the above described support means.
Furthermore, the above described guide means is configured by a wall surface of the process cartridge detachably mountable in the main body of the above described image forming apparatus.
In the image forming apparatus having the above described configuration, the conveyance regulating member is disposed so that a line connecting a sheet contact point on the conveyance regulating member to the conveying nip is directed above a tangential line on the conveying nip and when the pair of conveying members and the guide means are disposed so that a straight line passing the conveying nip and a vertex of the conveyance regulating member intersects with a conveying surface of the transferring means, the sheet is conveyed to the transfer position along the guide means and the leading end of the sheet strikes against the image bearing member at a small angle, whereby a striking impact of the sheet is weaker.
Furthermore, when the sheet trailing end regulating member is integrated with the guide means between the conveyance regulating member and the conveying nip, for example, it is possible to retard a sheet rising after the trailing end of the sheet has passed through the pair of conveying members, thereby stabilizing conveyance of the sheet.
Furthermore, when the sheet trailing end regulating member is made of an electrically insulating material such as resin, rubber or porcelain and charge eliminating means is disposed, for example, after the pair of conveying members, it is possible to insulate the charge eliminating means from the guide means at a bias voltage application time to the guide means, thereby enhancing a precision of image formation on the sheet.
Furthermore, when the guide means is integrated with the support means and the support means is hinged by the transfer roller shaft so as to swing, it is possible to enhance a positional precision of the guide means.
Furthermore, convenience of the image forming apparatus can be enhanced by utilizing a process cartridge which includes the image bearing member, etc. and which is detachably mountable in the main body of the image forming apparatus, and the image forming apparatus which has a simple configuration and can be manufactured at a low cost can be realized by using a wall surface of this process cartridge as the guide means.